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188                         METALLUKGY OF IKON AND STEEL.
carbon is estimated from the appearance of the fracture. The reliability of such a determination depends upon the constancy of the conditions of casting and chilling, and the expertness of the judge, but, roughly speaking, the content can be ascertained within 10 per cent, of the true amount.
SEC. Xj.—Recarburization.—When the desired point has been reached the recarburizer is added, being almost invariably used in a solid state. It is generally heated red hot, but this is not essential, for, in making structural steel, "ferro" containing 80 per cent, of manganese is used almost exclusively, and the weight of the addition is so small that it chilis the bath only slightly. The ferro may be added to the metal while in the furnace, and this method has the advantage that the bath can be thoroughly stirred after the recarburizer has melted, but it has the disadvantage that during the time the last pieces are fusing, the portions which melted first are losing their manganese to the oxygen of the slag and flame. In a hot furnace this action is very rapid, and although the entire addition may melt in less than a minute, a considerable proportion of manganese is lost by oxidation. When the recarburizer is added in the ladle, the latter action will not occur, but there will be a certain loss from the oxide of iron contained in the metal, and the function of the recarburizer is to remove this oxygen. The loss of manganese will be the same whether the addition is made in the furnace or in the ladle, but in the latter case the effects of slag and flame are absent. Hence, it follows that the loss will be more regular when recarburization is performed in the ladle, and the content of manganese in the steel more nearly alike throughout a series of heats.
The manganese lost in recarburization not only varies with the way in which it is added, but also with the percentage of carbon and manganese in the bath. The amount of oxide in the bath is less with high than with low carbons, and so the loss of manganese decreases as higher steel is made. Moreover, the loss is less with smaller percentages of manganese, so that if 1.00 per cent, of Mn be added there" will be .60 per cent, in the metal,'being a loss of .40 per cent., while if .50 per cent, be added the steel will have .40 per cent., being a loss of only .20 per cent. It seems as if with the lower manganese the action was not perfect, and that with each successive increment of ferro an additional atom of oxygen is re-